Water hyacinth is a free-floating perennialaquatic plant (or hydrophyte) native to tropical and sub-tropical South America. With broad, thick, glossy, ovate leaves, water hyacinth may rise above the surface of the water as much as 1 meter in height. The leaves are 10–20 cm across, and float above the water surface. They have long, spongy and bulbous stalks. The feathery, freely hanging roots are purple-black. An erect stalk supports a single spike of 8-15 conspicuously attractive flowers, mostly lavender to pink in colour with six petals. When not in bloom, water hyacinth may be mistaken for frog's-bit (Limnobium spongia).

One of the fastest growing plants known, water hyacinth reproduces primarily by way of runners or stolons, which eventually form daughter plants. Each plant can produce thousands of seeds each year, and these seeds can remain viable for more than 28 years.[1] Some water hyacinths were found to grow up to 2 to 5 metres a day in some sites in Southeast Asia.[2] The common water hyacinth (Eichhornia crassipes) are vigorous growers known to double their population in two weeks.

In their native range these flowers are pollinated by long tongued bees and they can reproduce both sexually and clonally. The invasiveness of the hyacinth is related to its ability to clone itself and large patches are likely to all be part of the same genetic form. There are three morphs of water hyacinth, long medium and short. However, the short morph is restricted to the native range due to founder events during its distribution.

Its habitat ranges from tropical desert to subtropical or warm temperate desert to rainforest zones. The temperature tolerance of the water hyacinth is the following; its minimum growth temperature is 12 °C (54 °F); its optimum growth temperature is 25-30 °C (77-86 °F); its maximum growth temperature is 33-35 °C (92-95 °F), and its pH tolerance is estimated at 5.0 to 7.5. It does not tolerate water temperatures >35 °C. Leaves are killed by frost and salt water, the latter trait being used to kill some of it by floating rafts of the cut weed to the sea. Water hyacinths do not grow when the average salinity is greater than 15% that of sea water. In brackish water, its leaves show epinasty and chlorosis, and eventually die.[3]

Because of E. crassipes invasiveness, several biological control agents have been released to control it, including two weevils (Coleoptera: Curculionidae), Neochetina bruchi Hustache and Neochetina eichhorniae Warner, and the moth Niphograpta albiguttalis (Warren) (Lepidoptera: Pyralidae).[4]Neochetina eichhorniae causes "a substantial reduction in water hyacinth production" (in Louisiana); it reduces plant height, weight, root length, and makes the plant produce fewer daughter plants. N. eichhorniae was introduced from Argentina to Florida in 1972.[5]

Azotobacter chroococcum, an N-fixing bacteria, is probably concentrated around the bases of the petioles. But the bacteria do not fix nitrogen unless the plant is suffering extreme N-deficiency.[6]

When not controlled, water hyacinth will cover lakes and ponds entirely; this dramatically impacts water flow, blocks sunlight from reaching native aquatic plants, and starves the water of oxygen, often killing fish (or turtles). The plants also create a prime habitat for mosquitos[citation needed], the classic vectors of disease, and a species of snail known to host a parasiticflatworm which causes schistosomiasis (snail fever).[10] Directly blamed for starving subsistence farmers in Papua New Guinea[citation needed], water hyacinth remains a major problem where effective control programs are not in place. Water hyacinth is often problematic in man-made ponds if uncontrolled, but can also provide a food source for goldfish, keep water clean [11][12] and help to provide oxygen[citation needed] to man-made ponds.

Water hyacinth often invades bodies of water that have been impacted by human activities[citation needed]. For example, the plants can unbalance natural lifecycles in artificial reservoirs or in eutrophied lakes that receive large amounts of nutrients.

The water hyacinth was introduced in 1884 at the World's Fair in New Orleans, also known as the World Cotton Centennial.[13] The plants had been given away as a gift by a group of visiting Japanese.[13] Soon after, the water hyacinth was choking rivers, killing fish and stopping shipping in Louisiana, and an estimated 50 kilograms per square meter choked Florida's waterways.[14] There were many attempts to eradicate the flower, including one by the U.S. War Department to pour oil over many of the flowers, but none worked.[13] In 1910, a bold solution was put forth by the New Foods Society. Their plan was to import and release hippopotamus from Africa into the rivers and bayous of Louisiana. The hippopotamus would then eat the water hyacinth and also produce meat to solve another serious problem at the time, the American meat crisis.[15]

Known as the American Hippo bill, H.R. 23621 was introduced by Louisiana Congressman Robert Broussard and debated by the Agricultural Committee of the U.S. House of Representatives.[16] The chief collaborators in the New Foods Society and proponents of Broussard's bill were Major Frederick Russell Burnham, the celebrated American scout, and Captain Fritz Duquesne, a South African scout who later became a notorious spy for Germany. Presenting before the Agricultural Committee, Burnham made the point that none of the animals that Americans ate, chickens, pigs, cows, sheep, lambs, were native to the U.S., all had been imported by European settlers centuries before, so why should Americans hesitate to introduce hippopotamus and other large animals into the American diet? Duquesne, who was born and raised in South Africa, further noted that European settlers on that continent commonly included hippopotamus, ostrich, antelope, and other African wildlife in their diets and suffered no ill effects. The American Hippo bill nearly passed, but fell one vote short.[15]

The plant was introduced by Belgian colonists to Rwanda to beautify their holdings and then advanced by natural means to Lake Victoria where it was first sighted in 1988.[17] There, without any natural enemies, it has become an ecological plague, suffocating the lake, diminishing the fish reservoir, and hurting the local economies. It impedes access to Kisumu and other harbors.

Water hyacinth has also invaded the Tonlé Sap lake in Cambodia. One project in Cambodia (Osmose) is trying to fight it by having local people make baskets from it . It was introduced in Bengal,India because of its beautiful flowers and shapes of leaves but turned out to be an invasive weed draining oxygen from the water bodies and resulted in death of many fishes. Fish is a supplement food in Bengal and due the fish scarcity produced in Bengal due to Eichhornia, it is also called as “Terror of Bengal”.[citation needed]

The three commonly used control methods against water hyacinth infestations are physical, chemical, and biological controls. No one control method is generally better than the others, because each has its advantages and disadvantages. The optimum control depends on the specific conditions of each affected location such as the extent of water hyacinth infestation, regional climate, and proximity to human and wildlife.[21]

Chemical control is the least used out of the three controls of water hyacinth, because of its long-term effects on the environment and human health. The use of herbicides requires strict approval from governmental protection agencies of skilled technician to handle and spray the affected areas. The use of chemical herbicides is only used in case of severe infiltration of water hyacinth.[22] However, the most successful use of herbicides is when it is used for smaller areas of infestation of water hyacinth. This is because in larger areas, more mats of water hyacinths are likely to survive the herbicides and can fragment to further propagate a large area of water hyacinth mats. In addition, it is more cost-effective and less laborious than mechanical control. Yet, it can lead to environmental effects as it can penetrate into the ground water system and can affect not only the hydrological cycle within an ecosystem but also negatively affect the local water system and human health. It is also notable that the use of herbicides is not strictly selective of water hyacinths; keystone species and vital organisms such microalgae can perish from the toxins and can disrupt fragile food webs.[21] The chemical regulation of water hyacinths can be done using common herbicides such as 2,4-D, glyphosate, and diquat. The herbicides are sprayed on the water hyacinth leaves and leads to direct changes to the physiology of the plant.[23] The use of the herbicide known as 2,4-D leads to the death of water hyacinth through inhibition of cell growth of new tissue and cellular apoptosis (Jimenez, 2005). It can take almost a two-week period before mats of water hyacinth are destroyed with 2,4-D. It has been found that up to 150,000 acres of water hyacinth are treated annually in Louisiana.[24] The herbicide known as diquat is a liquid bromide salt that can rapidly penetrate the leaves of the water hyacinth and lead to immediate inactivity of plant cells and cellular processes. For the herbicide glyphosate, it has a low toxicity than the other herbicides; therefore, it takes longer for the water hyacinth mats to be destroyed (about three weeks). The symptoms include steady wilting of the plants and a yellow discoloration of the plant leaves that eventually leads to plant decay.[22]

Physical control is performed by land based machines such as bucket cranes, draglines, or boorm or by water based machinery such as aquatic weed harvester,[25]dredges, or vegetation shredder.[26] Mechanical removal is seen as the best short-term solution to the proliferation of the plant. A project on Lake Victoria in Africa used various pieces of equipment to chop, collect, and dispose of 1500 hectares of water hyacinth in a 12-month period. It is, however, costly and requires the use of both land and water vehicles, but it took many years for the lake to become in poor condition and reclamation will be a continual process. It can have an annual cost from $6 million to $20 million and is only considered a short-term solution to a long-term problem. Another disadvantage with mechanical harvesting is that it can lead to further fragmentation of water hyacinths when the plants are broken up by spinning cutters of the plant-harvesting machinery. The fragments of water hyacinth that are left behind in the water can easily reproduce asexually and cause another infestation.[23] However, transportation and disposal of the harvested water hyacinth is a challenge because the vegetation is heavy in weight.The harvested water hyacinth can pose a health risk to humans because of the plant’s propensity for absorbing contaminants, and it be considered toxic to humans. However, the practice of mechanical harvesting is not effective in large-scale infestations of the water hyacinth, because this aquatic invasive species grows much more rapidly than it can be eliminated. In addition, only one or two acres of water hyacinth can mechanically harvested daily because of the vast amounts of water hyacinths in the environment. Therefore, the process is very time-intensive.[27]

In 2010 the insect Megamelus scutellaris was released by the Agricultural Research Service as a biological control for Water hyacinth.[28]

As chemical and mechanical removal is often too expensive and ineffective, researchers have turned to biological control agents to deal with water hyacinth. The effort began in the 1970s when USDA researchers released three species of weevil known to feed on water hyacinth into the United States, Neochetina bruchi, N. eichhorniae, and the water hyacinth borer Sameodes albiguttalis. The weevil species were introduced into the Gulf Coast states, such as Louisiana, Texas, and Florida, where there was thousands of acres of infestation from water hyacinth. It was found that a decade later in the 1980s that there was a decrease in water hyacinth mats by as much as 33%. However, because the life cycle of the weevils is only ninety days, it puts a limitation on the use of biological predation to efficiently suppress water hyacinth growth.[24] These organisms regulate water hyacinth by limiting water hyacinth size, its vegetative propagation, and seed production. They also carry microorganisms that can be pathological to the water hyacinth. These weevils eat stem tissue, which results in a loss of buoyancy for the plant, which will eventually sink.[23] Although meeting with limited success, the weevils have since been released in more than 20 other countries. However, the most effective control method remains the control of excessive nutrients and prevention of the spread of this species.

May 2010 the USDA’s Agricultural Research Service released Megamelus scutellaris as a biological control insect for the invasive waterhyacinth species. Megamelus scutellaris is a small planthopper insect native to Argentina. Researchers have been studying the effects of the biological control agent in extensive host-range studies since 2006 and concluded that the insect is highly host-specific and will not pose a threat to any other plant population other than the targeted water hyacinth. Researchers also hope that the biological control will be more resilient than existing biological controls to the herbicides that are already in place to combat the invasive water hyacinth.[29]

Another insect being considered as a biological control agent is the semi-aquatic grasshopper Cornops aquaticum. This insect is specific to the water hyacinth and its family, and besides feeding on the plant, it introduces a secondary pathogenic infestation. This grasshopper has been introduced into South Africa in controlled trials.[30]

Because of its extremely high rate of development, Eichhornia crassipes is an excellent source of biomass. One hectare of standing crop thus produce more than 70,000 m3 of biogas.[31] According to Curtis and Duke, one kg of dry matter can yield 370 liters of biogas, giving a heating value of 22,000 kJ/m3 (580 Btu/ft3) compared to pure methane (895 Btu/ft3)[32]

Wolverton and McDonald report only 0.2 m3 methane per kg, indicating requirements of 6000 MT biomass/ha to attain the 70,000 m3 yield projected by the National Academy of Sciences (Washington).[33] Ueki and Kobayashi mention more than 200 MT/ha/yr.[34] Reddy and Tucker found an experimental maximum of more than a half ton per day.[35]Bengali farmers collect and pile up these plants to dry at the onset of the cold season; they then use the dry water hyacinths as fuel. They then use the ashes as fertilizer. In India, a ton of dried water hyacinth yield circa 50 liters ethanol and 200 kg residual fiber (7,700 Btu). Bacterial fermentation of one ton yields 26,500 cu ft gas (600 Btu) with 51.6% methane, 25.4% hydrogen, 22.1% CO2, and 1.2% oxygen. Gasification of one ton dry matter by air and steam at high temperatures (800°) gives circa 40,000 ft3 (circa 1,100 m3) natural gas (143 Btu/cu ft) containing 16.6% hydrogen, 4.8% methane, 21.7% CO, 4.1% CO2, and 52.8% N. The high moisture content of water hyacinth, adding so much to handling costs, tends to limit commercial ventures.,[33][36] A continuous, hydraulic production system could be designed, which would provide a better utilization of capital investments than in conventional agriculture, which is essentially a batch operation.,[3][37]

The roots of Eichhornia crassipes naturally absorb pollutants, including lead, mercury, and strontium-90, as well as some organic compounds believed to be carcinogenic, in concentrations 10,000 times that in the surrounding water.[38] Water hyacinths can be cultivated for waste water treatment.[3]

Water hyacinth is reported for its efficiency to remove about 60–80 % nitrogen (Fox et al. 2008) and about 69% of potassium from water (Zhou et al. 2007). The roots of water hyacinth were found to remove particulate matter and nitrogen in a natural shallow eutrophicated wetland (Billore et al. 1998).[39]

Water hyacinth leaf extract has been shown to exhibit phytotoxicity against another invasive weedMimosa pigra. The extract inhibited the germination of Mimosa pigra seeds in addition to suppressing the root growth of the seedlings. Biochemical data suggested that the inhibitiory effects may be mediated by enhanced hydrogen peroxide production, inhibition of soluble peroxidase activity, and stimulation of cell wall-bound peroxidase activity in the root tissues of Mimosa pigra.[42]

In East Africa, water hyacinths from Lake Victoria are used to make furniture, handbags and rope.[43] The plant is also used as animal feed and organic fertilizer although there is controversy stemming from the high alkaline pH value of the fertilizer.[44] Though a study found water hyacinths of very limited use for paper production,[45] they are nonetheless being used for paper production on a small scale.

American-Nigerian Achenyo Idachaba has won an award for showing how this plant can be exploited for profit in Nigeria.[46]

In places where water hyacinth is invasive, overabundant, and in need of clearing away, these traits make it free for the harvesting, which makes it very useful as a source of organic matter for composting in organic farming in those locales, provided that the composting method properly handles it. As an aquatic plant, it requires most of the same composting principles as the seaweed that is composted close to sea coasts.

^Medicinal plants of east and southeast Asia. By L.M Perry. 1980. MIT Press, Cambridge. Cited in Handbook of Energy Crops. By J. Duke (Available only online. An excellent source of information on numerous plants.)